Hybrid automatic repeat request (HARQ) is a well known and widely used approach for mitigating disturbances of transmissions over various types of communication channels. For example, HARQ is applied to some signaling according to the communication standards advocated by the Third Generation Partnership Project (3GPP).
Different communication applications may have different latency requirements. Furthermore, different communication applications may, additionally or alternatively, have different requirements on reliability.
In a typical example (e.g. according to 3GPP), a control loop of a base station may strive to keep an error rate metric (e.g. a BLock Error Rate—BLER) at a predefined target value. The control loop may, for example, regulate the error rate metric towards the target value by adapting a transmission format (e.g. one or more of a modulation type, a coding rate, a block size, etc.). The target value is often chosen to optimize system throughput while accepting a number of retransmissions.
A typical BLER target value when HARQ is applied is 10%, which means that (on the average) 9 of 10 blocks will be delivered with low latency, since no retransmission is needed for those blocks.
However, 1 of 10 blocks (on the average) will be erroneous (i.e. not considered delivered) at the first reception, and at least one retransmission is needed for delivery of these blocks which increases the average latency.
The retransmitted blocks will have a much larger end-to-end delay than blocks that do not need retransmission due to the slow HARQ process involving signaling between the receiving unit and the transmitting unit. For applications with low latency requirements, frequently appearing HARQ retransmissions might not be acceptable.
If the delivery time for these blocks can be shortened, the average latency will be decreased. Furthermore, if the delivery time for these blocks can be shortened applications that are delay sensitive (e.g. having low latency requirements) may be handled using this approach.
“Pre-repeat selective-repeat ARQ in fading channel” by Nakamura, et al., IEEE Proceedings of International Conference on Universal Personal Communications, 1998 (ICUPC '98), Florence, Italy, Oct. 5-9, 1998, New York, N.Y., USA, pp. 1253-1257 discloses a selective-repeat ARQ scheme using a pre-repeat technique. Occurrence of errors in transmitted frames is predicted by observing errors in received frames when TDMA-TDD (time division multiple access, time division duplex) is employed. When error detection in a received slot predicts error in a previous transmitted slot, the content of the previous transmitted slot is pre-repeated.
A drawback with the approach disclosed in Nakamura, et al, is that a conclusion that a previously transmitted frame will reach its destination in error, where the conclusion is based on an erroneously received frame, may not be completely adequate in all scenarios. This may lead to unnecessary retransmissions (pre-repetitions), which may adversely affect throughput. Alternatively or additionally, it may lead to missed instances of transmitted frames in error, which may result in that normal retransmissions (non-pre-repetitions) will be applied and the delivery time will not be shortened.
Thus, there is a need for improved approaches that shorten the delivery time for blocks that need retransmission for correct delivery.